You shouldn't need to use pelleted fertilizer on freshly transplanted seedlings. I'm assuming you put them into some sort of shop bought compost so that should have enough nutrients to sustain the plants for at least a month to six weeks. Pelleted fertilizer is more for established plants that have been in the same place for some time as it slow releases the nutrients throughout the growing season. Although I'm not sure that it caused your plants to start dying as it would take a while to start to break down unless it was in direct contact with the roots then it could be the cause.